1. Field of the Invention
The field of the invention is combustion in a fluidized bed.
2. Description of Related Art
There are many fluidized bed combustion processes used commercially. Typical processes include the regenerators associated with FCC units, the regenerators used in fluid cokers, and combustion of coal, coke or similar substances in a dense bed to produce heat.
It is desirable to burn CO, produced by burning the coke or coal, to CO2 completely within the dense phase fluidized bed associated with the catalyst regenerator, fluid coker regenerator, or bed of coal. Complete combustion of CO to CO2 maximizes heat generation and minimizes air pollution.
Among the ways suggested to burn CO in FCC regenerators is to add a CO combustion promoter metal to the catalyst or to the regenerator. Metals have been added as an integral component of the cracking catalyst and as a component of a discrete particulate additive, in which the active metal is associated with a support other than the catalyst. U.S. Pat. No. 2,647,860 proposed adding 0.1 to 1 weight percent chromic oxide to a cracking catalyst to promote combustion of CO. U.S. Pat. No. 3,808,121, incorporated herein by reference, introduced relatively large-sized particles containing CO combustion-promoting metal into a cracking catalyst regenerator. The circulating particulate solids inventory, of small-sized catalyst particles, cycles between the cracking reactor and the catalyst regenerator, while the combustion-promoting particles remain in the regenerator. Oxidation-promoting metals such as cobalt, copper, nickel, manganese, copper-chromite, etc., impregnated on an inorganic oxide such as alumina, are disclosed.
U.S. Pat. Nos. 4,072,600 and 4,093,535 teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
Some work has been done on promoting carbon burning reactions by adding Pt coated sand to a bed of burning coke. Walsh and Green, in U.S. Pat. No. 4,515,092, which is incorporated herein by reference, reported that coke burning rates were increased, and that combustion efficiency improved as well. They used Pt on a relatively low surface area support, and found that fairly high Pt concentrations gave better results. Burning a small amount of solid fuel in a bed of sand, with no promoter being on the sand, gave a coke burning rate of 1.0 (base), while use of sand containing 1 wt % Pt increased the burning rate of the coke to 2.3. Essentially complete CO combustion to CO2 was always obtained, but the amount of Pt present was relatively large, 1 wt% of the fluidized bed, while the fuel was present in amounts ranging from 0.1 to 10 wt. % of the fluidized bed.
The patentees sought to avoid the problem of Pt loss by using a substrate which would remain in the fluidized bed.
Sand, and other low surface area materials were the preferred substrates for the Pt, but the possibility of using higher surface area supports, such as silica alumina, gamma alumina, and silica was mentioned, but no examples were provided on a support other than sand (80-240 mesh sand was used in the experiments).
Recovery of Pt on promoter from fines for recycle to the fluidized bed unit was not addressed.
Complete combustion of CO to CO2 in fluidized bed combustion will of course reduce CO emissions, but can cause as increase in NOx emissions. This has been the experience of FCC operators.
Many FCC units use CO combustion promoters. This reduces CO emissions, but usually increases nitrogen oxides (NOx) in the regenerator flue gas. It is difficult in a catalyst regenerator to completely burn coke and CO in the regenerator without increasing the NOx content of the regenerator flue gas.
Although many refiners have recognized the problem of NOx emissions from FCC regenerators, the solutions proposed have not been completely satisfactory. The approaches taken so far have generally been directed to special catalysts which will inhibit the formation of NOx in the FCC regenerator, or to process changes which reduce NOx emissions from the regenerator.
Recent catalyst patents include U.S. Pat. No. 4,300,997 and its division U.S. Pat. No. 4,350,615, both directed to the use of Pd-Ru CO-combustion promoter. The bi-metallic CO combustion promoter is reported to do an adequate job of converting CO to CO2, while minimizing the formation of NOx.
Another catalyst development is disclosed in U.S. Pat. No. 4,199,435 which suggests steam treating conventional metallic CO combustion promoter to decrease NOx formation without impairing too much the CO combustion activity of the promoter.
Process modifications are suggested in U.S. Pat. No. 4,413,573 and U.S. Pat. No. 4,325,833 directed to two-and three-stage FCC regenerators, which reduce NOx emissions.
U.S. Pat. No. 4,313,848 teaches countercurrent regeneration of spent FCC catalyst, without backmixing, to minimize NOx emissions.
U.S. Pat. No. 4,309,309 teaches the addition of a vaporizable fuel to the upper portion of a FCC regenerator to minimize NOx emissions. Oxides of nitrogen formed in the lower portion of the regenerator are reduced in the reducing atmosphere generated by burning fuel in the upper portion of the regenerator.
U.S. Pat. No. 4,235,704 suggests that too much CO combustion promoter causes NOx formation, and calls for monitoring the NOx content of the flue gases, and adjusting the concentration of CO combustion promoter in the regenerator based on the amount of NOx in the flue gas.
The approach taken in U.S. Pat. No. 4,542,114 is to minimize the volume of flue gas by using oxygen rather than air in the FCC regenerator, with consequent reduction in the amount of flue gas produced.
All the catalyst and process patents discussed above from U.S. Pat. No. 4,300,997 to U.S. Pat. No. 4,542,114, are incorporated herein by reference.
In addition to the above patents, there are myriad patents on treatment of flue gases containing NOx. The flue gas might originate from FCC units, or other units. U.S. Pat. No. 4,521,389 and U.S. Pat. No. 4,434,147 disclose adding NH3 to NOx containing flue gas and catalytically reducing the NOx to nitrogen.
None of the approaches described above provides the perfect solution. Process approaches which reduce NOx emissions require extensive rebuilding of the FCC regenerator.
Various catalytic approaches, e.g., use of bi-metallic CO combustion promoters, provide some assistance, but the cost and complexity of a bi-metallic combustion promoter is necessary. The reduction in NOx emissions achieved by catalytic approaches helps some but still may fail to meet the ever more stringent NOx emissions limits set by local governing bodies. Much of the NOx formed is not the result of combustion of N2 within the FCC regenerator, but rather combustion of nitrogen-containing compounds in the coke entering the FCC regenerator. Bi-metallic combustion promoters are probably best at minimizing NOx formation from N2.
I have discovered a way to overcome most of the deficiencies of the prior art methods of burning of coal or coke or similar substances in a dense phase fluidized bed. I have overcome several problems left unsolved by the prior art, particularly in the area of efficiency of use of the CO combustions promoters and in the area of reduced emissions of NOx. I use conventional CO combustion promoter metals on an unconventional catalyst support in an unconventional way.
By putting my Pt, or other CO combustion promoter, on a porous support, I can achieve complete CO combustion using orders of magnitude less Pt than was used in the experimental work reported in U.S. Pat. No. 4,515,092.
By putting the CO combustion promoter on a special support, I can achieve segregation of most of the CO combustion promoter within the upper portion of a dense phase fluidized bed and significantly reduce NOx emissions while maintaining satisfactory CO combustion. The approach was, in a sense, to turn the teaching of U.S. Pat. No. 3,808,121 upside down. The '121 patent added large-sized particles containing a CO combustion-promoting metal into an FCC regenerator. These particles because of their size and weight congregated at the bottom of the FCC regenerator dense bed. Withdrawal of hot regenerated catalyst occurred from an upper level of the FCC regenerator dense bed, so only the small-sized FCC catalyst cycled back and forth between the reactor.
In my process the CO combustion promoter preferably is concentrated in the upper portions of the dense phase fluidized bed.